The invention relates to a method of determining the production radiation dose in an apparatus for producing a pattern on a substrate provided in a substrate table, which method comprises the following steps:
providing a substrate having a radiation-sensitive layer in the substrate table; PA1 providing at least one substrate test mark in the radiation-sensitive layer by means of the production radiation; PA1 detecting a substrate test mark by means of an optical alignment device for aligning a substrate alignment mark with respect to a reference alignment mark; PA1 the substrate test mark having a periodical structure, with a period which is effectively equal to that of the reference alignment mark, and an asymmetrical division of irradiated and non-irradiated strips, the test mark detection consisting of:
first, aligning the substrate test mark relative to the reference alignment mark; and PA2 subsequently detecting a substrate test mark asymmetry interpreted by the alignment device as a substrate testmark offset, which asymmetry is dependent on the production radiation dose.
The invention also relates to a test mask which is particularly suitable for use in the method, and to an apparatus for producing a pattern in a substrate, with which the method can be performed.
Producing a pattern is herein understood to mean the provision of such a pattern, such as an IC pattern, in a radiation-sensitive layer. This may be realized by projecting a corresponding mask pattern in the radiation-sensitive layer, using a projection system, in the case of optical lithography, a projection lens system. An image of the mask pattern may also be realized by placing the mask close to the radiation-sensitive layer at a distance of several .mu.m and by illuminating this mask with production radiation. It is alternatively possible to write the test mark directly in the radiation-sensitive layer by scanning this layer with a narrow beam in conformity with the desired pattern.
The production radiation is the radiation which is used in the apparatus to provide a desired pattern, such as an IC pattern, in a production substrate, after checking measurements and tests, such as the radiation-dose measurement, have been performed. The production radiation may be electromagnetic radiation, such as deep UV radiation, which is used in an optical lithographic apparatus for repetitively imaging an IC mask pattern on a substrate in accordance with the step principle or in accordance with the step-and-scan principle. In the step method, an IC mask pattern is imaged on the substrate in a first IC area. Subsequently the substrate is moved with respect to the mask pattern until a second IC area of the substrate is present under the mask pattern and this pattern is imaged a second time. Then the substrate is moved again, and so forth, until an image of the mask pattern has been formed on all IC areas of the substrate. In the step-and-scan method, the IC pattern is not imaged in one operation but, for example, a narrow projection beam is used which each time images a part of the pattern corresponding to the beam cross-section, while both the mask pattern and the substrate are moved with respect to this beam until this beam has scanned the entire IC pattern and a complete image of the IC pattern has been formed in a first area of the substrate. Subsequently, the substrate is moved until a second IC area is situated under the mask pattern and the process of scanning-imaging is repeated, and so forth.
The production radiation may not only be an optical beam but also a charged-particle beam such as an electron beam or an ion beam with which, while using or not using a suitable projection system, an image of the mask pattern can be formed in a layer sensitive to this radiation, and in which such a beam in this layer causes material changes such as refractive index changes or chemical changes which can be converted into optically detectable changes.
It will be evident from the foregoing that the projection system may be an optical lens system, but also a different system such as an electron lens system which is used in electron beam imaging.
The article "Latent Image Metrology for Production Wafer Steppers" in SPIE, Vol. 2440, page 701, 1995 describes a method comprising the steps of the method described in the opening paragraph for determining the optimum focus setting of the projection lens system in a photolithographic apparatus and for determining various other parameters of the apparatus. In this article it is noted that, in principle, the detection signal obtained may be used to determine the optimum exposure dose, because the detection signal at a fixed focus setting changes with a change of the exposure dose. At the same time, it is also noted that an optimum test mark is being searched for this purpose.
As described in said article, a mask test mark, which is derived from a known mask alignment mark but is now asymmetrical, is imaged in the photosensitive layer of the substrate by means of the elements which are also used in the production process, namely the projection lens system and the production radiation, in the form of a projection beam. Then, a latent, i.e. undeveloped, image of this test mark is detected by means of the optical alignment device which is present in the apparatus.
The advantage of latent-image detection is that the image can be measured in the production apparatus itself and that the substrate with the substrate test mark does not need to be removed from the apparatus so as to be examined by means of, for example a scanning electron microscope. The advantage of the use of said test mark is that the apparatus does not need to be provided with a separate image detection device.
The test mark which is optimal for determining the optimum focus setting has a structure of strips and opaque intermediate strips, which corresponds to that of the alignment mark; however, the transmissive strips of the test mark have been replaced by strips which are partially transmissive and partially consist of a plurality of sub-strips which are alternately transmissive and non-transmissive to the projection beam radiation.
In this method, use is made of the fact that, while the alignment device determines that the center of one of the two symmetrical alignment marks coincides with the center of the image of the second symmetrical alignment mark in a situation where these two symmetrical alignment marks are aligned with each other, this device indicates that the center of an asymmetrical substrate test mark is offset with respect to the center, or point of gravity, of the symmetrical mask alignment mark in the situation where the asymmetrical substrate test mark is aligned with respect to this symmetrical mask alignment mark.
In principle, the above-mentioned asymmetrical test mark may also be used for determining exposure doses. However, it has been found that the exposure dose signal thus obtained is relatively weak and is not optimal for use in the production apparatus. It is an object of the present invention to optimize the radiation dose measurement based on the above-mentioned principle and to provide a suitable test mark. The method according to the invention is characterized in that, per period, a substrate test mark has at least two irradiated strips which are formed by means of the same number of different production radiation sub-doses.
Since within each period of the substrate test mark, two or more irradiated strips, which adjoin or do not adjoin each other and have received different radiation doses, are present in addition to a non-irradiated strip, this test mark is asymmetrical. It is ensured that the test mark effectively has the same period as the reference alignment mark of the alignment device, so that this test mark can be detected by means of this device and its asymmetry can be determined. Effectively the same period for the test mark and the reference mark is understood to mean that the period of the test mark, multiplied by the possible magnification factor with which this mark is imaged on the reference mark, is equal to the period of the reference mark. Use is made of the fact that an order filter is used in the alignment device, which, of the radiation from the test mark, only passes the radiation diffracted in the first orders by the test mark to the detectors. The alignment device supplies sine-shaped signals whose period is determined by that of the reference alignment mark. When observing the test mark, the phase shift of such a sine-shaped signal is measured, which shift is dependent on the asymmetry in the test mark.
As far as the provision of the test mark in the substrate is concerned, there are different embodiments of the method according to the invention. A first embodiment is characterized in that a substrate test mark is provided by directly inscribing this mark into the radiation-sensitive layer by means of narrow-beam charged-particle radiation.
A lithographic apparatus using ion, electron or X-ray radiation as production radiation is to this end provided with an optical alignment device, such as is used by the firm of ASM Lithography in its photolithographic apparatus which is known as wafer stepper. Since very small details can be written with the charged-particle radiation, this radiation is also suitable for directly inscribing the substrate test mark. This radiation causes local material changes in the sensitive layer on the substrate, which changes result in optical changes such as refractive index changes. The pattern of the changes can be observed with the optical alignment device which is provided with a reference alignment mark effectively having the same period as this pattern.
A second embodiment of the method is characterized in that a substrate test mark is provided by imaging a mask in which at least one mask test mark and one mask alignment mark are present, the mask alignment mark constituting the reference alignment mark and the mask test mark consisting of a periodical structure of intermediate strips which are alternately transmissive and non-transmissive to the production radiation and have a period which is equal to that of the mask alignment mark, the strips having a width of at most one fourth of the period, and in that imaging is realized by forming n sub-images of the mask test mark, while the mask and the substrate are movedwith respect to each other in between the successive sub-images in a direction perpendicular to the direction of the strips and along a distance which is at least equal to the width of the irradiated strips in the radiation-sensitive layer, the n sub-images being formed with n different radiation sub-doses and n being an integer of at least two.
Since in each radiation dose measurement the mask test mark with the relatively narrow transmissive strips is imaged n times, a substrate test mark is obtained which has the same period as the mask alignment mark but an exhibits asymmetry in the received radiation distribution and hence an asymmetry in the change of the material. When changing the radiation dose for the total image of the mask test mark, there will be a change of this asymmetry, in other words, the center of gravity of, for example, the refractive index distribution in the substrate test mark is displaced.
This method may be further characterized in that, for imaging the mask test mark, the mask is placed proximate to the substrate and the mask is irradiated with production radiation.
A proximity copy of the mask test mark is then made in the radiation-sensitive layer on the substrate.
This embodiment may be alternatively characterized in that, for imaging the mask test mark, use is made of a projection system which is arranged between the mask and the substrate.
Then, the mask test mark is projected in the radiation-sensitive layer of the substrate. In photolithography, a projection image is preferably used because reduced images of the production mask pattern can then be formed, so that the details in the mask pattern are larger than the corresponding details in the substrate. Such a mask pattern can be realized more easily and at a lower cost than a mask pattern which is suitable for a one-to-one image which is made with a proximity copy. This of course also applies to the mask test mark.
As already noted, when the alignment device detects the substrate test mark, a change of the asymmetry in this mark is interpreted as an offset of this mark relative to a reference. This reference is obtained in that, before detection of the test mark, for example, the substrate and the mask are accurately aligned with respect to each other by means of the global alignment marks which are already present for this purpose in the mask and the substrate and by means of the same alignment device, and in that the test mark is subsequently displaced towards the alignment beam while carrying out an accurate displacement measurement and check by means of a multi-axis interferometer system which is also already present in an optical lithographic apparatus for determining the movements of the substrate table relative to the mask table. By comparing the detected position of the test mark image with said reference, the apparent displacement of the test mark image acquires the effect of a zero offset of the alignment signal. This zero offset is dependent on the production radiation dose.
The novel method is not only suitable for latent images but may also be used to great advantage for developed images which have been converted into phase structures in the developing process. The detection of developed images is particularly important when using photosensitive layers which are specially suitable for radiation at a wavelength in the deep ultraviolet range, with which IC mask images having very small line widths of the order of 0.25 micron can be realized.
The term latent image comprises both the pattern of material changes obtained only by radiation, such as refractive index changes, and such a pattern which has been heated after irradiation, so that stronger material changes are produced via chemical reactions, resulting in stronger optical effects. The heated latent image is referred to as Post-Exposure Baking (PEE) image.
A preferred embodiment of the method, in which a substrate test mark is obtained by projection of a mask test mark, is characterized in that n is equal to two and that between the formations of the sub-images the mask and the substrate are moved with respect to each other along a distance which is equal to one fourth of the period of the substrate test mark.
A substrate test mark is then obtained whose irradiated strips are as wide as the non-irradiated strips, so that this mark has the same geometry as the alignment mark but an asymmetry in the received radiation distribution.
The number of images is then minimal, as well as the time required for a radiation dose measurement, while the measurement is still sufficiently accurate. n may alternatively be 3, 4, 5, etc. With an increasing n, a finer tuning of the radiation distribution is possible and a more accurate measurement can be obtained, but the time required for a radiation dose measurement also increases.
To be able to determine the correct radiation dose by way of comparison, the method according to the invention is further characterized in that the same test mark is provided m times at different positions on the substrate, while m different radiation doses each having n radiation sub-doses are used for forming the m test marks, and in that the correct radiation dose is determined by comparing the asymmetry in the m substrate test marks.
This method may be further characterized in that the ratio between the received radiation sub-doses is constant for the m substrate test marks.
All n radiation sub-doses then vary in conformity with a variation in the radiation doses of the m images.
Alternatively, this method may also be characterized in that one of the radiation sub-doses for the m substrate test marks varies, whereas the other radiation sub-doses are constant.
There are different embodiments of the method according to the invention, also as far as the test mark detection is concerned. A first embodiment of the method is further characterized in that, after the provision of the test mark in the radiation-sensitive layer, the latent image formed in this layer is detected by means of the alignment device.
This provides the possibility of a rapid measurement of the radiation dose.
A second embodiment of the method is further characterized in that, after the provision of the test mark in the radiation-sensitive layer, the substrate is removed from the substrate table, subsequently developed and then placed on a substrate table again, whereafter the developed substrate test mark is detected by means of the alignment device.
Detector signals having a large amplitude can be obtained in this way.
Both embodiments have the advantage that the substrate test marks are measured in the same or similar apparatus as the apparatus with which the test marks have been formed, and that the measurement can be carried out more rapidly than in the case where an optical or electron microscope would be used. It is alternatively possible to provide the substrate test marks in a first apparatus and to detect them in a second apparatus of the same type.
An embodiment of the method, with which a better reference for the test mark signal is obtained, is further characterized in that the radiation-sensitive layer is provided with a double mark consisting of said test mark and an associated alignment mark having a periodical structure of irradiated strips alternating with non-irradiated strips of the same width and a period which is equal to that of the test mark.
Since the alignment mark from which the reference is derived is located proximate to the test mark, the reference will be considerably more reliable than if it is derived from an alignment mark which is present at a larger distance from the test mark.
If the substrate test marks obtained by projection of a mask test mark are only to be used for a limited number of measurements, the method according to the invention may be further characterized in that use is made of a production mask which is provided with at least one test mark.
For obtaining a large degree of freedom in the choice of the positions of the test-mark images on the substrate, and hence in measuring possibilities, the method according to the invention is further characterized in that use is made of a test mask which is provided with at least one test mark.
The invention also relates to a novel test mask intended for use in the method described hereinbefore. This test mask, which is provided with at least one test mark and at least one alignment mark, in which the alignment mark and the test mark have a periodical structure of strips which are transmissive to the production beam radiation and alternate with opaque intermediate strips, with the periods of both structures being equal, is characterized in that the transmissive strips of the test mark have a width of at most one fourth of a period.
Said alignment mark may be formed by a global alignment mark which is also present in a production mask outside the IC pattern to be projected for aligning the mask with respect to the substrate. By making use of this alignment mark and the very accurate interferometer system for displacing the substrate table in a defined manner, the test mark in the aligned state can be introduced into the measuring beam of the alignment device.
A more accurate alignment and a more rapid detection of the test mark may, however, be realised if the test mask is further characterized in that a separate alignment mark of said type is provided proximate to a test mark.
Proximate is herein understood to mean that the distance between the alignment mark and the test mark at the level of the substrate is smaller than the dimension of an IC area, or in the case of projection lithography, is equal to a fraction of the dimension of the image field of the projection lens. If the projection lens exhibits distortion, then it is substantially the same for the images of both marks.
The test mark may be further characterized in that it comprises not only a test mark in the center but also in at least the four corners.
This provides the possibility of measuring the uniformity of the radiation throughout an IC area on a substrate and, in the case of projection lithography, the uniformity within the image field of the projection lens.
A further embodiment of the test mask is characterized in that each test mark comprises a plurality of portions, in which the direction of the grating strips of one portion is perpendicular to the direction of the grating strips of another portion.
With such a test mask, it is possible to measure in two mutually perpendicular directions.
In accordance with a further aspect, the test mask is further characterized in that the test mark has such a size that its image formed with the production radiation fits in an intermediate area on the substrate which is located between two areas in which the production mask pattern must be imaged.
A test mask which is particularly suitable for a short-lasting radiation dose measurement is further characterized in that the width of the strips for the test mark is equal to one fourth of the structure period.
The invention also relates to an apparatus for performing the method according to the invention, which apparatus successively comprises a radiation source unit for supplying a production radiation, and a substrate table, and further comprises an optical alignment device for aligning a substrate mark with respect to a reference alignment mark and a radiation dose measuring device. This apparatus is characterized in that the radiation dose measuring device is constituted by the alignment device and in that the radiation dose measuring device is adapted to detect, during each radiation dose measurement, the image of both a substrate test mark and of a substrate alignment mark and is provided with means for determining the difference between the observed aligned positions of the two mark images.
This apparatus may be further characterized in that it is further provided with a mask table for accommodating a test mask, and with a projection system arranged between the mask table and the substrate table.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.